1. Field of the Invention
The present invention relates to a method of etching a programmable memory microelectronic device and to a programmable memory electronic device obtained by said method.
2. Description of Related Art
Programmable memory microelectronic devices are typically, but not exclusively, programmable ionic conduction (metallization) cells, which are computer memories known as “non-volatile” computer memories. Such programmable ionic conduction cells are known by the acronyms CBRAM, standing for “conductive-bridging random access memory”, or PMC, for “programmable metallization cell”.
That type of microelectronic structure (CBRAM or PMC) is described in Document U.S. Pat. No. 6,084,796, for example.
A CBRAM (or PMC) typically comprises a vertical stack of layers formed by a substrate based on a silicon type semiconductor on which the following layers are deposited in succession: an electrode termed the bottom electrode, a layer of a chalcogenide glass doped with silver (i.e. solid electrolyte), and an electrode termed the top electrode formed from silver. The layer of a chalcogenide glass is interposed between the bottom electrode and the top electrode.
Said electrodes are configured to cause a metallic dendrite to grow (i.e. formation of an electrically-conductive bridge) from the negative of the two electrodes towards the positive of the two electrodes through the layer of doped chalcogenide glass when a voltage is applied between said electrodes. By applying an opposite voltage between said two electrodes, the reverse phenomenon is obtained, namely disappearance of the metallic dendrite (i.e. disappearance of the electrically-conductive bridge) within the doped chalcogenide glass layer.
Thus, when the electrically-conductive bridge is created (the step known as “writing”), the logic state of the device may be represented by “1”, or may correspond to the “ON” state, while when the electrically-conductive bridge disappears, the logic state of the cell may be represented by “0” or may correspond to the “OFF” state.
In order to produce the motifs for the CBRAM microelectronic structure, etching of the top electrode and the doped chalcogenide glass is routinely carried out using a hard mask. Said etching is conventionally carried out at high temperature in a metal etching chamber using a plasma.
However, numerous defects appear after said etching step, not only at the surface of the layers not covered by the hard mask, known as the “open surfaces”, but also at the ends of the etched layers.
Said defects are linked to the various metallic elements contained in the microelectronic layers of the CBRAM structure, such as the silver contained in the chalcogenide glass layer and in the top electrode. Said metallic elements are non-volatile elements, but are chemically highly reactive and highly mobile.
More particularly, the metallic defects at the surface of the open layers originate from vertical migration of said metallic elements in the CBRAM stack. In addition, the metallic defects at the ends of the etched layers are the cause of horizontal migration of metallic elements in the CBRAM stack.
Said metallic defects significantly degrade the quality of microelectronic CBRAM structures and thus limit their electrical performance or even render them non-functional.